BR40 in a BR30 fixture as long as the wattage is under the manufacturer recommended? It just fits nicer .

If the glass bulb size is the same so you get the same amount of free air circulation within the fixture, and the lamp base is in the same base-up or base-down configuration, I can't imagine a reason why you can't.
Of course, UL and the manuf. has the final word on this.

If the glass bulb size is the same so you get the same amount of free air circulation within the fixture, and the lamp base is in the same base-up or base-down configuration, I can't imagine a reason why you can't.
Of course, UL and the manuf. has the final word on this.